Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik® Aseptic, which is formed by folding and sealing a web of laminated packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with heat-seal plastic material, e.g. polyethylene.
In the case of aseptic packages for long-storage products such as UHT milk, the packaging material comprises a layer of oxygen-barrier material, e.g. a sheet of aluminium, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material eventually forming the inner face of the package contacting the food product.
As is known, such packages are made on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine itself, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the web of packaging material so sterilized is maintained in a closed sterile environment, and is folded and sealed longitudinally to form a tube.
The tube is fed continuously in a first vertical direction, is filled with the sterilized or sterile-processed food product, and is gripped and heat sealed at equally spaced cross sections by means of a heat-seal device.
More specifically, the heat-seal device comprises two or more pairs of jaws, which act cyclically and successively on the tube and heat seal the packaging material of the tube to form a continuous strip of pillow packs connected to one another by respective transverse pressure pads, i.e. extending in a second direction perpendicular to said first direction.
The pillow packs are separated by cutting the respective pressure pads, and are then fed to a final folding station where they are folded mechanically into the finished parallelepiped shape.
The tube portion gripped between each pair of jaws is heat sealed by heating means carried by one of the jaws and for locally melting the two layers of heat-sealable plastic material gripped between the jaws.
More specifically, packaging material in which the layer of barrier material is defined by a sheet of electrically conductive material, e.g. aluminium, is normally heat sealed by a so-called induction heat-seal process, in which, when the tube is gripped by the pair of jaws, electric current is induced in the sheet of aluminium to heat it locally and so locally melt the heat-sealable plastic material.
More specifically, in induction heat sealing, the heating means substantially comprise an inductor, which is carried by one of the two jaws, known as the sealing jaw, is supplied by a high-frequency current generator, and is substantially defined by one or more inducting bars made of electrically conductive material, extending parallel to the second direction, and which interact with and induce an electric current in the tube material to heat it to the necessary heat-seal temperature.
In the most common, known embodiment, the inductor defines, frontally, two straight, elongated active surfaces extending parallel to the second direction and on opposite sides of an intermediate plane perpendicular to the first direction. More specifically, the active surfaces are located in respective front contact surfaces of the sealing jaw, extending on opposite sides of and parallel to said intermediate plane and separated by a central recess, and have continuous or segmented longitudinal projections projecting towards the packaging material.
The other jaw, known as the counter-jaw, has two pressure pads made of elastomeric material, extending on opposite sides of and parallel to the intermediate plane, and having respective flat front contact surfaces, which cooperate with the respective front contact surfaces of the sealing jaw, and therefore with the respective active surfaces of the inductor, to heat seal the tube along respective sealing lines defining the transverse pressure pad. Once the heat-seal operation is completed, a cutting member, carried, for example, by one of the two jaws, normally the counter-jaw, is activated and which interacts with the tube of packaging material to cut the tube along the centreline of the transverse pressure pad between the sealing lines, and so cut a pillow pack off the bottom end of the tube of packaging material. The bottom end being sealed transversely, the jaws, on reaching the bottom dead-centre position, can be opened to avoid interfering with the top portion of the tube.
Though providing for good-quality seals, known heat-seal devices of the above type still leave room for improvement.
In particular, when packaging pourable food products containing small solid particles (e.g. fibres or seeds, as in tomato products), some of the particles may become lodged between the contacting portions of the two sheets of packaging material for sealing, thus locally impairing heat sealing of the packaging material. In which case, depending on the location of the particles trapped in the pressure pad, channels may be formed through the sealed portion, thus endangering the sterility of the package.
In EP 1,300,340 a pressure pad is shown having a constant, relatively small radius of the contact surface. This type of pad has been found to work well in displacing some of the particles from the sealing zone, but it can also cause undue stress to the packaging material.